Winter 2009 forecast for North America - update

Summary: Much of North America is expected to be colder than the 1971-2000 average. Positive temperature anomalies are forecast for the southern tier of the United States. The CF for a warm winter forecast is relatively high for the southeastern United States.

Fig. 1. The forecast of surface air temperature anomalies in North America in the winter (DJF) of 2009.  The blue (red) contours and the letter C (W) indicate colder (warmer) than normal temperatures. CL means near normal. The base period for calculating anomalies is 1971-2000. The numbers at the contours are confidence factors of the forecast.

Discussion

Changes from the previous forecast are most notable for the southeastern United States were a warmer than normal winter is now expected. These changes are related to a revision of the forecast for the North Atlantic. A positive phase of the North Atlantic Oscillation (NAO), which is now considered more likely for the winter of 2009, is strongly correlated with anomalously warm winters in the Southeast. This forecast is also supported by a negative phase of the Pacific Decadal Oscillation (PDO), which is expected to continue to dominate during the forthcoming winter (see the forecast for the North Pacific). A combination of these two powerful factors increases the confidence that the winter season in the Southeast will be overall warmer than average. However, an outbreak of cold air is expected to reach the Southwest in January, when a low pressure center may establish in the Gulf of Alaska (rule 198) and the atmospheric circulation will be characterized by a positive Pacific-North American (PNA) pattern (rule 227).